A tool for selective enabling of fresh not-as-stable software from specific,
topical RPM repositories for Amazon Linux.

This consists of a Catalog of topics, many Repositories, and this tool.

The Catalog is a JSON description of the topics available that a user can
subscribe to. For instance, "golang1.8" or "mysql3.45+php9.1" are topics, and
those may involve unnamed libraries and adapters and bridges and configuration, 

The Catalog has a version number "1", an optional message-of-the-day, a
dict of topics, and whitelists of configurations that Amazon knows to be able
to cohabitate on a computer.  topics is a list of dicts with "n" (topic name) and
"inst" (packages to install automatically). whitelists are list of integers
representing zero-based index into topics list to show topics we know can
co-habitate.  A set of topics are allowed to be enabled if there exists a
whitelist that is equal or is a superset of the proposed topics.

The underlying packages that make up a topic may change over time, when new
versions are released. A user could track changes and upgrade with DNF or YUM,
but it is expected most EC2 users will launch new machines with new images and
get newer packages newly installed, instead of upgrading existing packages.
Amazon makes no warranty about upgrading package versions.

The Code

The code is a thin wrapper in bin/ over a python module in
amazon_linux_extras/ . The wrapper overrides the default IO encoding
to be UTF-8 (not ASCII), sets up gettext and logging, and then calls main()
in the module.

The module adds subcommands by the presence of functions starting with "cmd_".
There are a few commands: list, enable, install, disable, help.

Enable, install, and disable write YUM/DNF "ini" configuration to a new file in 
/etc/yum.repo.d/ .

The names of topics need to be tweaked to be acceptable YUM repo ini sections,
so there are encode/decode functions for that. 

The "install" subcommand also runs DNF or YUM after enabling the repos.